The present invention relates to a process and an apparatus for the preparation of a polymer solution.
Polymers have been used in various technical fields. A polymer material such as a plastic film is formed by using a melt or solution of a polymer. A process of forming a polymer material comprises dissolving a polymer in a solvent to form a solution, forming a polymer material by using the solution, and drying the formed material by evaporating the solvent.
The solvent of a polymer is a liquid that can dissolve a polymer at a required concentration. The solvent also requires safety and an appropriate boiling point for evaporating the solvent from a formed polymer material. Recently, the solvent particularly requires safety of the human body and the environment. Therefore, it is now rather difficult to find an appropriate solvent in liquids that can dissolve a polymer.
For example, methylene chloride has been used as a solvent for cellulose triacetate. However, the use of hydrocarbon halides such as methylene chloride has recently been severely restricted to protect the global environmental conditions. Further, methylene chloride may cause problems in the workplace environment.
On the other hand, acetone is a widely used organic solvent. Acetone has an appropriate boiling point (56xc2x0 C.). Further, acetone has few adverse reactions with the human body and on global environmental conditions, compared with the other organic solvents. However, cellulose triacetate has poor solubility in acetone. Cellulose triacetate can swell in acetone, but scarcely dissolves in acetone.
J. M. G. Cowie et al. report in Makromol., Chem., 143 (1971) 105-114, that cellulose acetate having a substitution degree in the range of 2.70 (acetic acid content: 60.1%) to 2.80 (acetic acid content: 61.3%) is dissolved in acetone by a specific process. The process comprises the steps of cooling the cellulose acetate in acetone to a temperature of xe2x88x9280 to xe2x88x9270xc2x0 C., and warming it to obtain 0.5 to 5 wt. % solution of the cellulose acetate in acetone. The method of cooling a mixture of a polymer and a solvent to obtain a solution is hereinafter referred to as a cooling dissolution method.
The solution of cellulose acetate in acetone is also reported by K. Kamide et al., Textile Machinery Society, Vol. 34, 57-61 (1981). The report (written in Japanese) is entitled xe2x80x9cDry spinning process using acetone solution of triacetyl cellulose.xe2x80x9d In the report, the cooling dissolution method is applied to the art of fiber spinning. The experiments shown in the report examine the mechanical strength, the dyeing property and the cross sectional profile of the fiber obtained by the cooling dissolution method. In the report, 10 to 25 wt. % solution of cellulose acetate is used to form a fiber.
An object of the present invention is to dissolve a polymer in a solvent according to an improved cooling dissolution method, even if the polymer is swelled in, but is not dissolved in the solvent by a conventional dissolution method.
Another object of the invention is to provide an apparatus that can be advantageously used in a cooling dissolution method.
The present invention provides a process for the preparation of a polymer solution which comprises the steps of: mixing a polymer with a solvent to swell the polymer in the solvent; cooling the swelled mixture to a temperature of xe2x88x92100 to xe2x88x9210xc2x0 C. at a rate of faster than 1xc2x0 C. per minute; and then warming the cooled mixture to a temperature of 0 to 120xc2x0 C. to dissolve the polymer in the solvent.
The present invention also provides a process for the preparation of a polymer solution which comprises the steps of: mixing a polymer with a solvent to swell the polymer in the solvent; cooling the swelled mixture to a temperature of xe2x88x92100 to xe2x88x9210xc2x0 C.; and then warming the cooled mixture to a temperature of 0 to 120xc2x0 C. at a rate of faster than 1xc2x0 C. per minute to dissolve the polymer in the solvent.
The invention further provides an apparatus for the preparation of a polymer solution which comprises a stirring device, a cooling device connected to the stirring device, and a warming device connected to the cooling device, wherein both of the cooling device and the warming device include a rotary screw.
The invention furthermore provides an apparatus for the preparation of a polymer solution which comprises a stirring device, an extrusion device connected to the stirring device, a cooling device connected to the extrusion device and a warming device connected to the cooling device, wherein the extrusion device is a fiber or membrane extruding die, and both of the cooling device and the warming device mainly consist of a vessel.
A polymer can be dissolved in a solvent by a cooling dissolution method, even if the polymer is not dissolved in the solvent by a conventional dissolution method. It has been considered that the effect of the cooling dissolution method is obtained by a change of a molecular structure of a polymer molecule (e.g., destruction of an ordered structure of the molecule), which is caused by cooling and warming the polymer.
The present inventors have studied the cooling dissolution method, and have found that a polymer can more easily be dissolved in a solvent by cooling or warming the polymer quickly after swelling the polymer in the solvent. The inventors consider that a molecular structure of a polymer molecule is greatly changed by cooling or warming the polymer quickly (preferably cooling and warming the polymer quickly).
J. M. G. Cowie et al. are silent with respect to the cooling rate and the warming rate, except that acetone solutions of cellulose acetate are cooled to about 240xc2x0 K. and allowed to warm up at a rate of approximately 0.2xc2x0 per minute at a preliminary experiment of J. M. G. Cowie et al. K. Kamide et al. describe that the polymer cooled at xe2x88x9270xc2x0 C. is warmed to 50xc2x0 C. for 5 hours. Accordingly, the warming rate described in K. Kamide et al. is 0.4xc2x0 C. per minute. The cooling rate (which is not described in J. M. G. Cowie et al. and K. Kamide et al.) is supposed to be analogous to the warming rate.
In the process of the present invention, the swelled mixture is cooled at a rate of faster than 1xc2x0 C. per minute or the cooled mixture is warmed at a rate of faster than 1xc2x0 C. per minute. According to the process of the invention, a polymer solution can be made from various combinations of polymers and solvents. Accordingly, the number of solvents that can dissolve a polymer is increased by the present invention. Therefore, an appropriate solvent can be selected from many liquids that dissolve a polymer according to the process of the invention.
The process of the invention can be advantageously conducted by using an apparatus of the present invention, which can quickly cool or warm a mixture of a polymer and a solvent. The apparatus of the invention has another advantage of an excellent thermal efficiency.